The invention relates in general to the field of high power linear amplification and in particular to a technique of controlling the operation of a LINC style RF amplifier.
Many applications exist that require high power radio frequency (RF) amplifiers capable of linear operation while at the same time operate at high efficiency. Linear operation is desirable due to the presence of amplitude modulation on many types of input signals and regulatory requirements to minimize spurious or out of channel response. Efficiency is desirable due to the economics of minimizing thermal loading and operational costs. Normally, achieving the combination of linear operation and high efficiency has proven difficult.
An approach to implementing a high power amplifier that is both linear and efficient was first proposed by Chireix in 1935 as described in H. Chireix, xe2x80x9cHigh Power Outphasingxe2x80x9d, Proc. IRE, Vol. 23, No. 11, November 1935, pp 1370-1392. Chireix""s concept was based on an outphasing approach in which the input signal is decomposed into a pair of constant amplitude signals, which are then amplified by a pair of efficient, possibly non-linear, RF amplifiers, and then linearly combined to form the high power output. Cox later revived this concept in the early 1970""s and coined the term LINC (Linear Amplification Using Non-Linear Components) by which this amplifier design concept is popularly known today. See D. C. Cox, xe2x80x9cLinear Amplification with Nonlinear Componentsxe2x80x9d, IEEE Trans. Comm., December 1974, pp 1942-1945.
The LINC amplifier has a LINC modulator which decomposes an input signal into two or more constant-amplitude phase-modulated components. Each component is then amplified in a separate channel, by a phase-preserving high power amplifier (HPA, which may otherwise be nonlinear). A power combiner is also provided to combine the amplified components of the different channels, resulting in a linearly amplified version of the input signal. To improve overall linearity, the accuracy of the LINC modulator may be enhanced by implementing it using digital signal processing. See, e.g. S. A. Hetzel, A. Bateman, J. P. McGeehan, xe2x80x9cLINC Transmitterxe2x80x9d, pp 844-846, Electronic Letters, Vol. 27. No. 9, May 9, 1991; L. Sundstrom, xe2x80x9cEffects of Reconstruction Filters and Sampling Rate For A Digital Signal Component Separator On LINC Transmitter Performancexe2x80x9d, Electronic Letters, pp 1124-1125, Vol. 31, No. 14, Jul. 6, 1995; L. Sundstrom, xe2x80x9cThe Effect of Quantization In A Digital Signal Component Separator For LINC Transmittersxe2x80x9d, IEEE Trans. Vehicular Technology, pp 346-352, Vol. 45, No. 2, May 2, 1996. Linearity is also improved by balancing the frequency response of the channels in which the components are amplified. This has been done using automatic digital equalization filters in one or more of the channels, which compensate the components for any expected imbalance between the channels that might cause distortion at the power combiner output. See e.g. WO 96/19063, xe2x80x9cMethod of Balancing the Channels of a LINC Amplifierxe2x80x9d, Jun. 20, 1996; U.S. Pat. No. 5,886,673 xe2x80x9cAmplification Using Amplitude Reconstruction of Amplitude and/or Angle Modulated Carrierxe2x80x9d, Mar. 23, 1999; U.S. Pat. No. 5,990,738, xe2x80x9cCompensation System and Methods for a Linear Power Amplifierxe2x80x9d, Nov. 23, 1999; U.S. Pat. No. 6,215,354, xe2x80x9cClosed Loop Calibration for an Amplitude Reconstruction Amplifierxe2x80x9d. However, changing operating conditions internal to the amplifier can alter the overall gain of the conventional LINC amplifier. This means that the overall, or net, gain of the amplifier is varying during operation, which is an undesirable characteristic.